1. Field of the Invention
The present invention relates to an arrangement for a cable or any other line member with respect to a robot arm. The present invention also relates to an industrial robot to which such a cable arrangement is applied.
2. Description of the Related Art
Arc-welding robots are equipped with a welding wire feeder so that supplementary filler wire (consumable electrode), unwound from a wire reel, is fed to the welding torch. At the torch, the wire is melt with electric energy to perform welding continuously. The wire feeder is provided with presser rollers for forwarding the filler wire under pressure to the welding torch.
Arc-welding robots typically have multiple joints. For example, the entire arm mechanism includes a total of six joints, namely, first through sixth joints, and these joints are individually operated for causing the operational arm to rotate, swing, pivot or tilt. As a result, the end effecter can manipulate the welding torch. The wire feeder is mounted on a tilt table which supports the rotational upper arm and is caused to pivot at one end of a lower arm (the upper end of the lower arm when held in the upright position). The pivotal movement of the tilt table is implemented by the operation of the relevant joint, which is typically the third joint among the six joints.
The wire reel and the wire feeder are connected with each other by a cable, and the wire feeder and the welding torch are connected with each other by another cable. The former cable is a conduit pipe for guiding the movement of the filler wire. The latter is a multi-layered single-line power cable (torch cable), including a conduit pipe for the filler wire, an outside layer through which shielding gas is supplied, another outside layer serving as a power line, and an outermost insulating sheath.
Because of the multi-layer structure, the torch cable tends to be rigid and therefore is difficult to handle. Conventionally, in an attempt to alleviate the difficulty in manipulating the welding torch, the torch cable is made long enough to be amply slack. Specifically, in order to render the cable flexible, the cable from the wire feeder to the welding torch is placed outside of the arm, making sure that appropriate deformation is possible when the welding torch is caused to face in the desired direction (upward, downward, to the right or left, etc.).
In operating a welding robot, the operability or movable range of the welding torch is limited by the length and flexibility of the torch cable, for example. As is often the case, the torch cable can be worn out in repeated bending action of the arm, which is a problem of the prior art. Another problem is that when the welding arm in motion comes to a sudden stop, for instance, the torch cable tends not to stop together with the welding arm, but continues to swing due to its inertia. Such undesired movement disturbs smooth feeding of the welding wire through the conduit pipe. Thus, conventionally, the arm acceleration and deceleration must be controlled at the sacrifice of the mobility of the robot.
Further, the conventional torch cable arrangement causes the following problem. When a welding robot is operated with other robots, jigs, etc. placed around, or is used for welding an area inside of a container (which may be cylindrical or box-like, for example), the torch cable may come into direct interference with the things around or the side walls of the container, which results in poor operation efficiency and poor quality of products.
For the purposes of increasing the movable range of the welding torch and improving the welding quality, several proposals have been conventionally made. For example, JP-A-S62-140794 discloses a 3-joint wrist utilizing a hollow driving shaft within which a cable, an air pipe or a paint pipe is arranged to extend along the longitudinal axis. As another example, JP-A-H02-155572 discloses an arrangement that a torch cable runs at the center of each joint.
As described above, the conventional torch cable is routed along the longitudinal axis of the arm so as to pass the center of a driving shaft or a joint. This arrangement is adopted because it is thought to minimize the torch cable deformation even under a circumstance where the joints are repeatedly operated.
However, such an idea can only hold true when the multi-joint robots use their joints primarily in rotating or twisting movements but not much for swinging or tilting movements. The arrangement is not always ideal when the multi-joint robot has to work with much bending operation, or keep a bending attitude. In the case of an arc-welding robot for example, the welding torch must often be set or maintained in a predetermined attitude in order to create a good molten pool which determines welding quality. In a 6-axis (6-articulated) robot for example, an arm member at the fifth shaft (fifth joint) needs to be rotated through e.g. 120 degrees at maximum.
In this case, if the arrangement is to lay the torch cable to pass the center of driving shaft or the joint, closely along the longitudinal axis of the rotation arm, the torch cable, after passing the center of the fourth shaft, will deform by gradually leaving the longitudinal axis of the rotation arm in an upward direction, and then bending downward along the fifth shaft, into the shape of a question mark ‘?’. When the torch cable is deformed into a question mark, stress tends to accumulate at places of the conduit pipe where the torch cable rises from the rotation arm longitudinal axis and where it bends sharply downward. The stress accelerates the wear-out of the torch cable.
Since the conduit pipe has a larger inner diameter than that of the filler wire, curvature of the conduit pipe and curvature of the filler wire are not identical with each other. The curvature difference causes the filler wire to rub against the conduit pipe at least two locations, i.e. before and after the bend. The rubbing location changes as the curvature varies, and the friction force acting between the conduit pipe and the filler wire varies as the conduit pipe deforms, which results in unstable feeding speed.
Likewise, as the fifth shaft varies its posture or the amount of bend (tilting angle), the movement influences the upstream torch cable. Specifically, curvature of the cable changes between the fourth shaft and the fifth shaft, which results in difference in the length of conduit pipe and the length of filler wire. The amount of change in the length of conduit pipe is compensated by the same amount of sliding movement of the filler wire in or out of the welding torch. This results in undesirable variation in the amount of wire supply, disturbing the welding arc, making impossible to achieve significant improvement in the welding quality.